This invention relates to cylindrical magnetrons and specifically to methods and systems for sealing endblocks used in cylindrical magnetrons. The cylindrical magnetron is used in a large coating machine for coating very large sheets of glass or other materials. One application where these sheets of glass are used is in construction of curtain wall buildings where a single glass sheet can be up to 15 feet wide by about 20 plus feet high. The sheets are run through the coating machine shortly after the glass is manufactured. Thus, these are large-scale machines, which must rapidly and evenly coat glass as quickly as it can be manufactured. In addition to the quality of the coating the magnetron deposits upon the glass, dependability and serviceability of the magnetron is of the utmost importance.
This is not an easy task taking into account the constraints of the process that is involved. A cylindrical magnetron sputters material from a rotating target tube onto the glass as it is transported past the target. In order to coat such a large piece of glass the target tube can be up to 15 feet in length and 6 inches in diameter and can weigh 1700 pounds. Another complication is that the sputtering actually erodes the target tube during the sputtering process, so the target tube is constantly changing shape during its serviceable lifetime. The sputtering process can require that an extremely high AC or DC power (400 amps, 150 kW) be supplied to the target. This power transfer creates extreme heat in the target tube and the surrounding components, which must be cooled in order assure proper performance and to avoid catastrophic failure of the magnetron. Thus, water is pumped through the center of the rotating target tube at high pressure and flow rate.
Rotating such a large target tube in such an environment is a difficult task. FIG. 1A depicts magnetron 100 for illustrative purposes. FIG. 1B shows magnetron 100 integrated into a large glass coating system 130. Glass coating system 130 maybe several hundred feet long and contain many magnetrons. Target tube 106 is supported by two endblocks 104 and 108 as glass sheet 110 passes by. The endblocks 104 and 108 generally supply cooling water, support and rotate the target tube, support a stationary magnetic array within the rotating target tube, and transfer the large amounts of electricity needed for the sputtering process. Effectively transferring electrical power to a rotating target tube is also a complex problem. Maintaining electrical isolation in a sputtering process is also crucial to continually laying down a uniform coating on the glass. If the drive system is not properly electrically isolated from the sputtering process, it will affect the quality of coating deposited upon the glass. The sputtered material may in fact also coat the drive and electrical components of the magnetron itself rather than the glass if they are not properly isolated. Aside from resulting in a poor coating, this has many other ramifications on the continuous reliable operation of the magnetron. For further information please refer to “Coated Glass Application and Markets” by Russell J. Hill and Steven J. Nadel, The BOC Group, 1999 (ISBN #0-914289-01-02).
Efficient and effective sputtering also requires that the process take place in a vacuum or a reduced pressure relative to atmosphere. One or more vacuum pumps may be connected to provide vacuum within a coater. Thus, endblocks must have a very robust sealing system to prevent air or high-pressure water from leaking into the vacuum environment as the target is rotated. Typically sputtering takes place at a pressure of 2×10−3 Torr and the chamber may be pumped to a base pressure of about 2×10−6 Torr.
Maintaining a good seal around a rotating part may be achieved using lip seals that seal against the rotating surface. However, such seals suffer from wear over time and may eventually fail allowing air to leak past the seal. Such a leak may not be detected in time to prevent damage to products in the coater.
Therefore, there is a need for a robust sealing system that allows a vacuum seal to be maintained between a stationary part and a rotating part and that allows early detection of failure of a seal. There is also a need for a method of determining which seal has failed where multiple seals are present in a coater.